1. Field of the Invention
The present invention relates to a method and apparatus for quantitatively determining a concentration of a trace component mixed in a gas.
2. Description of Related Art
In a quantitative analysis method of gas concentration using a Fourier transform infrared spectrophotometer, when quantitative analysis is conducted of the concentration of a trace component gas in a sample gas, it is an important task to improve measurement precision. In this connection, it is required to conduct an accurate measurement without any interference by drift and noise.
Accordingly, the following will discuss, for example, a method of calculating an absorbance spectrum Abs of ammonia gas selected as a sample gas and water vapor selected as a trace component gas [1]. There are obtained a single beam spectrum SB(Samp) of the ammonia gas, and a single beam spectrum SB(BG) of background gas (e.g., nitrogen gas) which is not absorbed in a measurement wavenumber region of water vapor. Then, an absorbance spectrum Abs is calculated based on these two types of single beam spectra.
[Patent document 1] Japanese Unexamined Patent Publication 2002-22536.
According to this method, however, it is required to use a background gas having a moisture content removed therefrom by 100%, but it is not possible to obtain a background gas of which moisture content is removed by 100%. Even though nitrogen gas of six-nine (6N) purity is used, moisture content of 0.01 ppm is contained. It is, therefore, difficult to conduct a quantitative analysis of a trace moisture content in the sample gas. Further, the conditions under which the sample gas is measured cannot be made completely identical with those under which the background gas is measured. Accordingly, there remains a drift factor such as temperature variations over time.